Introduction to Syntax
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Transcript of Introduction to Syntax
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What is Syntax?
• Study of structure of language• Specifically, goal is to relate surface
form (e.g., interface to phonological component) to semantics (e.g., interface to semantic component)
• Morphology, phonology, semantics farmed out (mainly), issue is word order and structure
• Representational device is tree structure
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What About Chomsky?• At birth of formal language theory (comp sci) and
formal linguistics • Major contribution: syntax is cognitive reality• Humans able to learn languages quickly, but not all
languages universal grammar is biological• Goal of syntactic study: find universal principles
and language-specific parameters• Specific Chomskyan theories change regularly• These ideas adopted by almost all contemporary
syntactic theories (“principles-and-parameters-type theories”)
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Types of Linguistic Activity
• Descriptive: provide account of syntax of a language; often good enough for NLP engineering work
• Explanatory: provide principles-and-parameters style account of syntax of (preferably) several languages
• Prescriptive: “prescriptive linguistics” is an oxymoron
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Structure in Strings
• Some words: the a small nice big very boy girl sees likes
• Some good sentences:o the boy likes a girl o the small girl likes the big girlo a very small nice boy sees a very nice boy
• Some bad sentences:o *the boy the girlo *small boy likes nice girl
• Can we find subsequences of words (constituents) which in some way behave alike?
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Structure in StringsProposal 1
• Some words: the a small nice big very boy girl sees likes
• Some good sentences:o (the) boy (likes a girl) o (the small) girl (likes the big girl)o (a very small nice) boy (sees a very nice boy)
• Some bad sentences:o *(the) boy (the girl)o *(small) boy (likes the nice girl)
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Structure in StringsProposal 2
• Some words: the a small nice big very boy girl sees likes
• Some good sentences:o (the boy) likes (a girl) o (the small girl) likes (the big girl)o (a very small nice boy) sees (a very nice boy)
• Some bad sentences:o *(the boy) (the girl)o *(small boy) likes (the nice girl)
• This is better proposal: fewer types of constituents
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More Structure in StringsProposal 2 -- ctd
• Some words: the a small nice big very boy girl sees likes
• Some good sentences:o ((the) boy) likes ((a) girl) o ((the) (small) girl) likes ((the) (big) girl)o ((a) ((very) small) (nice) boy) sees ((a) ((very) nice)
girl)
• Some bad sentences:o *((the) boy) ((the) girl)o *((small) boy) likes ((the) (nice) girl)
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From Substrings to Trees
• (((the) boy) likes ((a) girl))
boythe
likesgirl
a
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Node Labels?
• ( ((the) boy) likes ((a) girl) )• Choose constituents so each one has one
non-bracketed word: the head• Group words by distribution of constituents
they head (part-of-speech, POS):o Noun (N), verb (V), adjective (Adj), adverb (Adv),
determiner (Det)• Category of constituent: XP, where X is POS
o NP, S, AdjP, AdvP, DetP
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Node Labels
• (((the/Det) boy/N) likes/V ((a/Det) girl/N))
boy
the
likes
girl
a
DetP
NP NP
DetP
S
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Types of Nodes
• (((the/Det) boy/N) likes/V ((a/Det) girl/N))
boy
the
likes
girl
a
DetP
NP NP
DetP
S
Phrase-structuretree
nonterminalsymbols= constituents
terminal symbols = words
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Determining Part-of-Speech
o noun or adjective? a child seat a blue seat *a very child seat *this seat is child It’s a noun!
o preposition or particle? he threw the garbage out the door *he threw the garbage the door out he threw out the garbage he threw the garbage out
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Word Classes (=POS)
• Heads of constituents fall into distributionally defined classes
• Additional support for class definition of word class comes from morphology
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Some Points on POS Tag Sets
• Possible basic set: N, V, Adj, Adv, P, Det, Aux, Comp, Conj
• 2 supertypes: open- and closed-classo Open: N, V, Adj, Advo Closed: P, Det, Aux, Comp, Conj
• Many subtypes:o eats/V eat/VB, eat/VBP, eats/VBZ, ate/VBD,
eaten/VBN, eating/VBG, o Reflect morphological form & syntactic
function
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Phrase Structure and Dependency Structure
likes/V
boy/N girl/N
the/Det a/Detboy
the
likes
girl
a
DetP
NP NP
DetP
S
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Types of Dependency
likes/V
boy/N girl/N
a/Detsmall/Adjthe/Det
very/Adv
sometimes/Adv
ObjSubjAdj(unct)
FwFw
Adj
Adj
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Grammatical Relations
• Types of relations between wordso Arguments: subject, object, indirect
object, prepositional objecto Adjuncts: temporal, locative, causal,
manner, …o Function Words
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Subcategorization
• List of arguments of a word (typically, a verb), with features about realization (POS, perhaps case, verb form etc)
• In canonical order Subject-Object-IndObj
• Example:o like: N-N, N-V(to-inf)o see: N, N-N, N-N-V(inf)
• Note: J&M talk about subcategorization only within VP
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Where is the VP?
boy
the
likes
girl
a
DetP
NP NP
DetP
S
boy
the
likesDetP
NP
girl
a
NP
DetP
S
VP
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Where is the VP?
• Existence of VP is a linguistic (i.e., empirical) claim, not a methodological claim
• Semantic evidence???• Syntactic evidence
o VP-fronting (and quickly clean the carpet he did! )o VP-ellipsis (He cleaned the carpets quickly, and so
did she )o Can have adjuncts before and after VP, but not in
VP (He often eats beans, *he eats often beans )• Note: in binary branching, it is
methodological; also in certain CFGs
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Context-Free Grammars
• Defined in formal language theory (comp sci)
• Terminals, nonterminals, start symbol, rules
• String-rewriting system• Start with start symbol, rewrite
using rules, done when only terminals left
• NOT A LINGUISTIC THEORY, just a formal device
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CFG: Example• Many possible CFGs for English, here is an
example (fragment):o S NP VPo VP V NPo NP DetP N | AdjP NPo AdjP Adj | Adv AdjPo N boy | girlo V sees | likeso Adj big | smallo Adv very o DetP a | the
the very small boy likes a girl
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Derivations in a CFG
S NP VPVP V NPNP DetP N | AdjP NPAdjP Adj | Adv AdjPN boy | girlV sees | likesAdj big | smallAdv very DetP a | the
S
S
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Derivations in a CFG
S NP VPVP V NPNP DetP N | AdjP NPAdjP Adj | Adv AdjPN boy | girlV sees | likesAdj big | smallAdv very DetP a | the
NP VP
NP
S
VP
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Derivations in a CFG
S NP VPVP V NPNP DetP N | AdjP NPAdjP Adj | Adv AdjPN boy | girlV sees | likesAdj big | smallAdv very DetP a | the
DetP N VP
DetP
NP
S
VP
N
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Derivations in a CFG
S NP VPVP V NPNP DetP N | AdjP NPAdjP Adj | Adv AdjPN boy | girlV sees | likesAdj big | smallAdv very DetP a | the
the boy VP
boythe
DetP
NP
S
VP
N
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Derivations in a CFG
S NP VPVP V NPNP DetP N | AdjP NPAdjP Adj | Adv AdjPN boy | girlV sees | likesAdj big | smallAdv very DetP a | the
the boy likes NP
boythe likes
DetP
NP
NP
S
VP
N V
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Derivations in a CFG
S NP VPVP V NPNP DetP N | AdjP NPAdjP Adj | Adv AdjPN boy | girlV sees | likesAdj big | smallAdv very DetP a | the
the boy likes a girl
boythe likes
DetP
NP
girla
NP
DetP
S
VP
N
N
V
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Derivations in a CFG;Order of Derivation
Irrelevant
S NP VPVP V NPNP DetP N | AdjP NPAdjP Adj | Adv AdjPN boy | girlV sees | likesAdj big | smallAdv very DetP a | the
NP likes DetP girl
likes
NP
girl
NP
DetP
S
VP
N
V
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Derivations of CFGs
• String rewriting system: we derive a string (=derived structure)
• But derivation history represented by phrase-structure tree (=derivation structure)!
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Grammar Equivalence
• Can have different grammars that generate same set of strings (weak equivalence)o Grammar 1: NP DetP N and DetP a | theo Grammar 2: NP a N | NP the N
• Can have different grammars that have same set of derivation trees (strong equivalence)o With CFGs, possible only with useless ruleso Grammar 2’: DetP many
• Strong equivalence implies weak equivalence
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Normal Forms &c
• There are weakly equivalent normal forms (Chomsky Normal Form, Greibach Normal Form)
• There are ways to eliminate useless productions and so on
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Generative Grammar
• Formal languages: formal device to generate a set of strings (such as a CFG)
• Linguistics (Chomskyan linguistics in particular): approach in which a linguistic theory enumerates all possible strings/structures in a language (=competence)
• Chomskyan theories do not really use formal devices – they use CFG + informally defined transformations
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Nobody Uses CFGs Only (Except Intro NLP Courses)
• All major syntactic theories (Chomsky, LFG, HPSG, TAG-based theories) represent both phrase structure and dependency, in one way or another
• All successful parsers currently use statistics about phrase structure and about dependency
• Derive dependency through “head percolation”: for each rule, say which daughter is head
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Massive Ambiguity of Syntax
• For a standard sentence, and a grammar with wide coverage, there are 1000s of derivations!
• Example:o The large head painter told the
delegation that he gave money orders and shares in a letter on Wednesday
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Penn Treebank, Again
• Syntactically annotated corpus (phrase structure)
• PTB is not naturally occurring data!• Represents a particular linguistic theory
(but a fairly “vanilla” one)• Particularities
o Very indirect representation of grammatical relations (need for head percolation tables)
o Completely flat structure in NP (brown bag lunch, pink-and-yellow child seat )
o Has flat Ss, flat VPs
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Types of syntactic constructions
• Is this the same construction?o An elf decided to clean the kitcheno An elf seemed to clean the kitchen An elf cleaned the kitchen
• Is this the same construction?o An elf decided to be in the kitcheno An elf seemed to be in the kitchenAn elf was in the kitchen
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Types of syntactic constructions (ctd)
• Is this the same construction?There is an elf in the kitcheno *There decided to be an elf in the
kitcheno There seemed to be an elf in the
kitchen
• Is this the same construction?It is raining/it rainso ??It decided to rain/be rainingo It seemed to rain/be raining
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Types of syntactic constructions (ctd)
Conclusion: • to seem: whatever is embedded
surface subject can appear in upper clause
• to decide: only full nouns that are referential can appear in upper clause
• Two types of verbs
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Types of syntactic constructions: Analysis
to seem: lower surface subject raises to upper clause; raising verb
seems there to be an elf in the kitchenthere seems t to be an elf in the kitchenit seems (that) there is an elf in the kitchen
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Types of syntactic constructions: Analysis
(ctd)
• to decide: subject is in upper clause and co-refers with an empty subject in lower clause; control verb
an elf decided an elf to clean the kitchenan elf decided to clean the kitchenan elf decided (that) he cleans/should clean the
kitchen*it decided (that) he cleans/should clean the
kitchen
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Lessons Learned from the Raising/Control Issue
• Use distribution of data to group phenomena into classes
• Use different underlying structure as basis for explanations
• Allow things to “move” around from underlying structure -> transformational grammar
• Check whether explanation you give makes predictions